Surely its sale (government portion) would depend on the lifespan of the battery? Do they even know how long these giant batteries will actually last being relatively new technology?

The battery is expected to last 20-30 years (or 10,000 cycles..charges and recharges).

Sorry no sources for this assertion, but I have read this in the past.

I am not ideologically opposed to the sale of the government section of the battery (especially it the proceeds were used to "seed" future storage options)
but my guess is that the function would stay the same.

The computers at Hornsdale monitor the National Electricity Market with intimate knowledge of who is producing how much power and where.

This is what makes the battery so useful in "grid stabilization"...it is like a "big brother" watching the grid, instantly adding power when "fluctuations" occur.

The health of the grid is number one priority, easily more important than power shortfalls in any given area.

I thought state Labor claimed the batteries wiped costs of nearly $50 million per year for frequency stability and grid restart services.

I think I read that the batteries respond quickly to instability and so the grid operators do not need to pay as much or as often to the gas generators to keep things stable. I think this is a side-effect and is not built into the current grid services pricing model - which means that there is no financial or contract arrangement in place to ensure that the batteries do it, rather than doing it for free (most of the time) and just getting a polite "thank you".

Great article from Renew Economy detailing the Tesla's battery response to an interstate power issue.
If you didn't understand why the battery is so important then you will after you read this.

How the Tesla big battery kept the lights on in South Australia

They say that lightning never strikes twice. But on August 25 last year a single lightning strike managed to take out two major circuits on the main transmission line linking NSW and Queensland.

The impact was almost immediate, and felt across Australia’s main grid. It caused load-shedding at a scale that made the much-talked about load shedding in Victoria in January’s heat wave look comparatively small beer.

We reported the events at the time, noting the irony of the Tesla big battery holding together the grid on the very first full day of work for the new prime minister, Scott Morrison, the man who had spent so much time promoting coal and ridiculing new technologies like the big battery.